1. Field of the Invention
The invention relates to a method of encapsulating critical rubber and plastic chemicals. More particularly, the invention relates to novel compositions comprising critical rubber or plastic chemicals and binders which mix rapidly and homogeneously with the rubber or plastic compositions to which they are added.
2. Description of the Prior Art
In the processing of rubber and plastic compositions, certain chemicals that play an important function in the final compound must be added in relatively small amounts, generally in the range of 0.1 to 5 percent by weight of the final compound although occasionally as high as 20 percent by weight. These chemicals must be added in certain critical proportions so that the desired processing conditions and/or physical properties of the finished or manufactured product are uniformly reproducible. Small variations in the amount or distribution of these chemicals in the final compound will cause serious variations in the processing and/or physical properties of the final compound. Because of the accuracy with which these chemicals must be added to rubber or plastic compositions, they have been termed "critical chemicals." Examples of critical chemicals include vulcanizing agents, curing agents, and cross-linking agents, accelerators and retardants for such agents, antidegradants, antioxidants, antiozonants, blowing agents, blowing agent activators, pigments, colorants, cure modifiers, flame retardants, chemical and heat stabilizers, reinforcing agents, resins, inhibitors, catalysts which extend polymer chains, and the like. Clay or oil, for example, would not be considered critical chemicals in a typical batch of 500 pounds of a rubber compound since they function as diluents and variations of 5 to 10 pounds from batch to batch would hardly be noticed. Variations of 3 or 4 ounces of accelerator or vulcanizing agent in a typical batch, however, can significantly change the physical properties of the final compound and/or its processing properties.
Prior to the present invention, various methods were employed for dispersing or encapsulating critical chemicals. One such method is master-batching. This is generally accomplished for both rubber and plastics on two-roll mills or in internal mixers, such as the Banbury type. The products so produced can subsequently be formed into shapes convenient for packaging and weighing. When two-roll mills are employed, dwell times are relatively long. Although labor costs are lower when internal mixers are used, higher temperatures and, therefore, more heat history are generally encountered. Furthermore, energy costs are high and the quality of chemical dispersion is generally inferior to the products of two-roll mills.
Another prior art method involves the formation of pastes of the critical chemicals. The binder compositions for such pastes include various oils and other plasticizers, low molecular weight, non-crystalline polymers, and waxes. The pastes can be prepared in apparatus such as mullers and three-roll mills, which can subsequently be formed into various shapes by extrusion or molding. U.S. Pat. No. 3,000,841 discloses the preparation of such pastes.
A further prior art method involves surface treatment. This procedure does not constitute true dispersion and generally accomplishes only partial encapsulation. Treatment is with various liquids, such as oils or propionic acid, and is carried out in apparatus such as ribbon blenders and Patterson-Kelly Twin Cone mixers.